docs: def as_bel(self) -> str:
          """Return this node as a BEL string."""
          variants = self.get(VARIANTS)
  
          if not variants:
              return super(CentralDogma, self).as_bel()
  
          variants_canon = sorted(map(str, variants))
  
          return "{}({}:{}, {})".format(
              self._func,
              self.namespace,
              ensure_quotes(self._priority_id),
              ', '.join(variants_canon)
          )